A resistive switching element is a circuit element whose electrical resistance is determined by its previous current-voltage history in such a way that an applied voltage of sufficient magnitude can switch it between a high-conductance state and a low-conductance state. The term “memristor” or the term “resistive random access memory” (ReRAM or RRAM) is often used to refer to a passive two-terminal resistive switching element fabricated with a thin insulating film sandwiched between two conductors.
Memristors are among the most promising emerging memory devices to replace flash, DRAM, and possibly SRAM. The desirable analog behavior of memristors may also make them a candidate for neuromorphic circuit applications. Currently known types of memristors are believed to work because large-signal voltages (typically >1 V) across the device may be used to shift the spatial distribution of the dopant ions in the insulator, which in turn acts to change the small-signal (typically <1 V) resistance of these devices.
The insulating materials used until now in memristors are generally transition-metal oxide (TMO) films containing mobile oxygen ions and oxygen vacancies, such as TiO2, Ta2O5, WO3, HfO2, NiO, and Nb2O5. Electrodes are typically made from electrically conductive metals. In many instances, a pair of metals is favored for the respective electrodes, in which one metal more readily reduces the insulator material than the other.
Materials choices may, however, be constrained by compatibility requirements when it is desired, e.g., to integrate memristors with CMOS integrated circuits. For that reason, there remains a need for further designs for memristors that use CMOS-compatible materials.